The malignant invasion of cancer cells, or metastasis, is the most common cause of cancer- related deaths, however the mechanisms that allow epithelial tumor cells to invade distant organs remain unclear. A cancer cell must overcome many obstacles on its journey to distant organs, and a better understanding of the metastatic cascade could turn these obstacles into opportunities for therapeutic intervention. It is widely believed that epithelial-mesenchymal transition (EMT) and mesenchymal epithelial-transition (MET) are critical for malignant spread because these processes dramatically alter the morphology and behavior of cancer cells, allowing them to adapt to new microenvironments. However, due to the difficulty of identifying cells that have undergone EMT and MET in vivo, the molecular mechanisms that facilitate these transitions and their significance during metastasis are not clear. The goal of this proposal is to define the molecular and cellular mechanisms of metastasis with a focus on EMT and MET. EMT results in invasive behavior in vitro, and is associated with high rates of metastasis and poor prognosis in many types of human carcinomas. Despite the correlation between EMT and metastatic spread, it is still not clear whether EMT is required for this process. MET is thought to be important for later steps in the metastatic cascade and is associated with increased lung colonization in mouse models of breast and bladder cancer~ however these studies do not address whether MET is necessary for metastasis. The molecular mechanisms that allow a cancer cell to undergo EMT and then revert back to an epithelial phenotype are not well understood but seem to be crucial for malignant progression. In this proposal I will investigate the role of epigenetic modifications in EMT and MET and determine whether these transitions are required for metastasis. First I will use an orthotopic transplant model and inducible cell ablation to determine if EMT is required for hematogenous dissemination, an early step in the metastatic cascade. Secondly I will use similar methods to determine if MET is required for the colonization of distant organs, a late step in metastasis. Lastly I will determine if altered histone modifications at epithelial and mesenchymal promoters are associated with EMT status in vivo. Together these experiments will define the roles of EMT and MET in metastasis and shed light on the in vivo regulation of these transitions.